You first need to create the progress indicator, which is quick and easy:
var progressIndicator = Ext.create("Ext.ProgressIndicator", {
loadingText: "Uploading: {percent}%"
});
Then in your XHR request, you'll set the progress property like so:
...
progress: progressIndicator
...
I would have put an example together for you myself, but I found one on GitHub here: https://github.com/senchalabs/ajax_demos/blob/master/ajax-upload.html
You may also want to check out this existing bug as it deals with file upload and effects versions <= 2.3.1: http://www.sencha.com/forum/showthread.php?275277-File-Upload-from-Sencha-Touch-2.3-app
Happy Coding!